CALIBRATION

Calibration is a comparison of instrument performance to the standards of known accuracy; calibrations directly link customers measurement equipment to national and international standards.

In other words, calibration means to find out whether the instrument gives the correct reading or not. It also includes minor adjustments in the instrument to minimize error.

VERNIER CALIPER

The vernier, dial, and digital calipers give a direct reading of the distance measured to high accuracy. They are functionally identical, with different ways of reading the result. These calipers comprise a calibrated scale with a fixed jaw, and another jaw, with a pointer, that slides along the scale. The distance between the jaws is then read in different ways for the three types.

The simplest method is to read the position of the pointer directly on the scale. When the pointer is between two markings, the user can mentally interpolate to improve the precision of the reading. This would be a simple calibrated caliper; but the addition of a vernier scale allows more accurate interpolation, and is the universal practice; this is the vernier caliper.

Vernier, dial, and digital calipers can measure internal dimensions (using the uppermost jaws), external dimensions using the lower jaws, and in many cases depth by the use of a probe that is attached to the movable head and slides along the centre of the body.

Parts of a vernier caliper: 1. Outside jaws: used to measure external diameter or width of an

object2. Inside jaws: used to measure internal diameter of an object3. Depth probe: used to measure depths of an object or a hole4. Main scale: scale marked every mm5. Main scale: scale marked in inches and fractions6. Vernier scale gives interpolated measurements to 0.1 mm or better7. Vernier scale gives interpolated measurements in fractions of an inch8. Retainer: used to block movable part to allow the easy transferring of

a measurement

VERNIER HEIGHT GAUGE

This is also a sort of vernier caliper equipped with a special base block & other attachments which makes the instruments suitable for height measurements. Along with the sliding jaw assembly, arrangement is provided to carry a removable clamp.The vernier height gauge is mainly used in the inspection of parts and layout work. This can also be used as scribing instrument.

Vernier height gauge

VERNIER DEPTH GAGE

6-7

Measures depths of holes, slots and recesses.

Read same as standard vernier caliper

MICROMETER

INTRODUCTION

Micrometer  allows a measurement of the size of a body.

It is one of the most accurate mechanical devices in common use.

PRINCIPLE OF MICROMETER

Micrometer works on the principle of screw and nut. We know that when a screw is turned through a nut through one revolution, it advances by one pitch distance i.e. one revolution of the screw corresponds to linear movement of a distance equal to pitch of the thread.

MICROMETER INTRODUCTION

The micrometer screw gauge consists of an accurate screw having about 10 or 20 threads per cm and revolves in a fixed nut. The end of the screw forms one measuring tip & the other measuring tip is constituted by a stationary anvil in the base of the frame. The screw is threaded for a certain Length & is plain afterwards. The plane portion is called sleeve & its end is the measuring surface. The spindle is advanced or retracted by turning a thimble connected lo the spindle. The spindle is slide fit over the barrel & barrel is the fixed path attached with the frame. Barrel is graduated in the unit of 0.005 cm. The thimble has got 25 divisions around its periphery on circular portion. Each division corresponds to 0,002 cm, A lock nut is provided for locking a dimension by preventing the motion of the spindle.

MICROMETER CALIPERS

MAIN PARTS OF MICROMETER

U-shaped steel frame:-- The outside micrometer has U or C shaped frame.

It holds all the parts of micrometer together . The frame is generally made of steel, cast steel,

malleable C. I. of light alloy. It is desirable that the frame of Micrometer be

provided with conveniently placed finger grips.

Anvil and Spindle: The Micrometer has fixed anvil protruding 3 mm

fro the left hand side of the frame. The diameter of the anvil is the same as that of the spindle.

Another movable anvil is provided on the front of the spindle. The anvil are accurately ground and lapped. The spindle engages with the nut.

Lock nut:  A lock nut is provided on the micrometer spindle.

It is used to lock the spindle when the micrometer is at its correct reading.

Sleeve or Barrel: The sleeve is accurately divided and clearly marks

in 0.5 mm division along its length, which serves as main scale.

It is chrome platted and adjustable for zero setting.

Thimble : Thimble can be moved over the barrel. It has 50

equal division around its circumference. Each division having a value of 0.01mm.

Ratchet: The ratchet is provided at the end of thimble. It is

used for accurate measurement.

THREAD MICROMETER CALIPER

This is same as a micrometer only the anvil & spindle is of different shape. The anvil has an internal V-shape which fits in the thread. The anvil in this case is not fixed but is free to rotate- Thus vee of the anvil can accommodate itself to any rake range of thread. The spindle on the other hand has the ground conical shape. When the conical spindle brought into contact with vee of anvil, micrometer reads zero. Different set of anvil is provided for different threads sizes.

Screw Thread Micrometer Caliper

DEPTH MICROMETER

• Depth gauge micrometers are used to measure the depth of blind holes, slots, key ways, etc.

•The spindle length can be changed to set the micrometer for the desired range of measurement.

•To read a depth gauge micrometer you must visualize the distance that has been covered by the thimble.

Depth Gauge Micrometer

INSIDE MICROMETER

Used for measuring cylinder bores, housing bores

Screw pitches same as outside micrometer

DIAL GAUGE

Dial gauges are used for checking flatness of surfaces and parallelism of bars and rods.

Used for linear measurement

Two pointer arms actuated by rack and pinion arrangement

Rack is cut in spindle and spindle is made to come in contact with the work-piece

Linear displacement is converted into rotary movement of the pointers.

The dial is divided into 100 equal divisions and each division represents a spindle movement of 0.01 mm.

For 1mm movement, the bigger arm turns through one complete revolution.

The smaller arm register the number of full turns made by bigger arm

THE CLINOMETER

CLINOMETERS

Cllinometer is a level mounted on a rotatable member whose angle of inclination relative to its base can be measured by a circular scale.

Used for checking angular faces and relief angle on large cutting tools.

For precision work watt clinometers are often employed (available in four types). The first type contains the front of a glass circular reading from 0-360 degree ,and used as similar to optical protractor as already used .The second type of instrument operates by means of a worm and quadrant reading direct to n1 seconds and has a range of 90 degree. The third type of instrument contains any bubble and their operation depends upon a circle supported on fine ball bearings and weighted eccentrically so that, when released, they always take up the position relative to the true vertical. The angle of the body is read of 1 second with the aid of vernier.

The fourth type of clinometers illustrated is one of high precision, reading direct to three seconds. dust tight body, mounted on an accurately ground base, carries on a lapped steel bearing, a sensitive spirit level which serves as a fiducially indicator

AUTO COLLIMATOR An autocollimator or auto collimated telescope is an instrument combining

a telescope and a collimator, such that the graticule and any superimposed image can be observed through the eyepiece. This instrument is designed to measure small angular deflections and may be used in conduction with plane mirror or other reflecting surface .

If a scale is provided on the graticule the tilt of the reflecting surface may be measured .The angle of the reflected ray ,relative to the incident ray, is twice the angle of tilt of the reflecting surface ,so that a direct two –to –one magnification is obtained .

AUTO COLLIMATOR

AUTO COLLIMATOR

MICROPTIC AUTOCOLLIMATOR

Most common method for calibrating surface plates. The instrument is used for measuring the straightness of

the long and relatively narrow surface such as beds of long lathes and planning machines

SINE BAR

Sine bars are always used along with slip gauges as a device for the measurement of angles very precisely.

They are used to1) Measure angles very accurately.2) Locate the work piece to a given angle with

very high precision. Generally, sine bars are made from high

carbon, high chromium, and corrosion resistant steel. These materials are highly hardened, ground and stabilized.

SINE BAR

In sine bars, two cylinders of equal diameter are attached at lie ends with its axes are mutually parallel to each other.

They are also at equal distance from the upper surface of the sine bar mostly the distance between the axes of two cylinders is 100mm, 200mm or 300mm.

The working surfaces of the rollers are finished to 0.2 μm R value.

The cylindrical holes are provided to reduce the weight of the sine bar

The working of sine bar is based on trigonometry principle

To measure the angle of a given specimen, one roller of the sine bar is placed on the surface plate and another one roller is placed over the surface of slip gauges.

Now, ‘h be the height of the slip gauges and ‘L’ be the distance between roller centers, then the angle is calculated as

USE OF SINEBAR: (1,) Locating any’ work to a given angle

Before checking the unknown angle of the specimen, first the angle (0) of given specimen is found approximately by bevel protractor.

2)Then the sine bar is set at angle of 0 and clamped on the angle plate.

3)Now, the work is placed on the sine bar and the dial indicator set at one end of the work is moved across the work piece and deviation is noted.

4)Slip gauges are adjusted so that the dial indicator reads zero throughout the work surface.

Limitations of sine bars 1)Sine bars are fairly reliable for angles than

15°. 2)It is physically difficult to hold in position

3)Slight errors in sine bar cause larger angular errors.

4)A difference of deformation occurs at the point of roller contact with the surface plate and to the gauge blocks.

5)The size of parts to be inspected by sine bar is limited

Sources of error in sine bars: The different sources of errors are listed below: 1)Error in distance between roller centers. 2)Error in slip gauge combination. 3)Error in checking of parallelism. 4)Error in equality of size of rollers and

cylindricity. 5)Error in parallelism of roller axes with each

other. 6)Error in flatness of the upper surface of sine

bar.

BEVEL PROTRACTORS

Bevel protractors are nothing but angular measuring instruments.

Types of bevel protractors The different types of bevel protractors used

are: 1)Vernier bevel protractor 2)Universal protractor 3)Optical protractor

. VERNIER BEVEL PROTRACTOR

A vernier bevel protractor is attached with acute angle attachment.

The body is designed its back is flat and no projections beyond its back. The base plate is attached to the main body and an adjustable blade is attached to the circular plate containing vernier scale.

The main scale is graduated in degrees from 0° to 90° in both the directions. The adjustable can be made to rotate freely about the center of the main scale and it can be locked at any position.

For measuring acute angle, a special attachment is provided. The base plate is made fiat for measuring angles and can be moved throughout its length. The ends of the blade are beveled at angles of 45° and 60°

The main scale is graduated as one main scale division is 1° and vernier is graduated into 12 divisions on each side of zero. Therefore the least count is calculated as

Thus, the bevel protractor can be used to measure to an accuracy of 5 minutes.

Applications of bevel protractor The bevel protractor can be used in the

following applications. 1.For checking a ‘V’ block:

COMPARATORS

INTRODUCTION A comparator works only on relative

measurements i.e. It gives only dimensional difference in relation to a basic dimension. so, a comparator has to compare the unknown dimensions of a part with some standard or master setting which represents the basic size and dimensional variations from the master setting have to be amplified and measured

CHARACTERISTICS OF COMPARATORS

1. The instrument must be of robust design and construction so as to withstand the effect of ordinary usage without impairing its measuring accuracy.

2. The indicating devices must be such that readings are obtained in least possible time. The system should be free from errors, wear effects and the inertia should be minimum possible.

3. Provision must be made for maximum compensation for temperature effects.

4. The scale must be linear 5. Indicator should be constant in its return to zero.

1. Instrument must have the maximum versatility, i.e., its design must be such that it can be used for a wide range of operations.

USES OF COMPARATORSUSES OF COMPARATORS

1. In mass production, where components are to be checked at a very

fast rate.

2. As laboratory standards from which working or inspection gauges

are set and correlated.

3. For inspecting newly purchased gauges.

4. Attached with some machines, comparators can be used as

working gauges to prevent work spoilage and to maintain required

tolerances at all stages of manufacturing.

5. In selective assembly of parts, where parts are graded in three or

more gauges depending upon their tolerance.

The mass production would be impossible if component parts could not be produced to close dimensional tolerances.

Example: An alluminium piston for a motor car engine

Very large quantity of this is required to produce and this means that piston does not only be mass produced, but also all dimensions must be checked with some kind of precision and speed as that used in their manufacture.

If the principle of measurement by comparison is adopted, say the height of the piston, then the set-up would appear as shown in figure

Comparators Are Needed Because• Determination of accuracy takes only a few seconds• Little or no skill is required from the operator • Consistency of measuring operation would be of a high

standard Elements of a Comparator Elements of a Comparator

1. Sensing device (usually a plunger): which senses the input

signal, may be a change of length or a surface

displacement.

2. Magnifying or Amplifying system: to increase the signal to a

suitable magnitude. Mechanical, optical, pneumatic, hydraulic and

electronic methods are used for this purpose.

3. Display system (usually a scale and pointer): which utilizes the

amplified signal to provide a suitable readout?

CLASSIFICATION OF COMPARATORS

• Mechanical comparators :

• Mechanical – optical comparators

• Electrical and Electronic comparators

• Pneumatic comparators

• Fluid displacement comparators

• Projection comparators

• Multi-check comparators

• Automatic gauging machines

MECHL MECHANICAL COMPARATORS :

‘In these comparators, magnification is obtained by mechanical linkages and other mechanical devices.’

Systems of Amplifications used in Mechanical Comparators:

1. Rack and Pinion: In it the measuring spindle integral with a rack, engages a pinion which amplifies the movement of plunger through a gear train.

2. Cam and gear train: In this case the measuring spindle acts on a cam which transmits the motion to the amplifying gear train.

3. Lever with toothed sector: In this a lever with a toothed sector at its end engages a pinion in the hub of a crown gear sector which further meshes with a final pinion to produce indication.

MECHANICAL COMPARATORS

Dial Indicator Read Type Mechanical Comparator

Dial Indicator:

READ TYPE MECHANICAL COMPARATOR:

Advantages of Mechanical Comparators:

1. These are usually cheaper in comparison to other devices of amplifying.

2. These do not require any external supply such as electricity or air and as such the variations in outside supplies do not affect the accuracy.

3. Usually the mechanical comparators have linear scale which is easily understood.

4. These are usually robust and compact and easy to handle.

5. For ordinary workshop conditions, these are suitable and being portable can be issued from a store.

Disadvantages of Mechanical Comparators:

1. The mechanical comparators have got more moving

parts than other types. Due to more moving parts,

the friction is more and ultimately the accuracy is

less.

2. The mechanism has more inertia and this may

cause the instruments to be sensitive to vibration.

3. The range of the instrument is limited as the pointer

moves over a fixed scale.

4. Error due to parallax is possible as the moving

pointer moves over a fixed scale.

OPTICAL COMPARATOR

There is no pure optical comparator but the instruments classed as optical comparators obtain large magnification by use of optical principles though mechanical magnification in these instruments also contributes for overall magnification.

There are many types of optical

comparators but all of them work on

one of the following two principles

1. The use of the optical lever

2. The use of enlarged image`

PRINCIPLE OF OPTICAL LEVER :-

In principle of optical lever, small displacements of the measuring plunger are amplified first by a mechanical system consisting of pivoted levers. The mechanical movement is further Amplified by a simple optical system involving the projection of an image. The usual arrangement employed is such that the mechanical system causes a plane reflector to tilt about an axis and the image of an index is projected on a scale on the inner surface of a ground- glass screen.

In this system, Mechanical amplification l2/l1 and optical amplification = l4/l3 ×2

It is multiplied by 2, because if mirror is tilted by an angle δθ, then image will be tilted by 2 × δθ. Thus overall magnification of this system = 2 × (l2/l1) (l4/l3)

TYPES OF OPTICAL COMPARATORS

Optical Projector Zeiss Optotest Comparator Eden - Rolt Millionth Comparator

ADVANTAGES OF OPTICAL COMPARATORS

1. It has small number of moving parts and

hence a higher accuracy.

2. In the optical comparators the scale can be

made to move past a datum line and thus

have high range and no parallax errors.

3. It has very high magnification.

4. Optical lever is weightless.

DISADVANTAGES OF OPTICAL COMPARATORS

1. As the instrument has high magnification, heat

from the lamp, transformer. May cause the

setting to drift.

2. An electrical supply is necessary.

3. The apparatus is usually large and expensive.

4. When the scale is projected on the screen, then

it is essential to use the instrument to a dark

room in order to take the readings easily.

5. The instruments in which the scale is viewed

through the eye piece of a microscope are not

convenient for continuous use.

ELECTRICAL COMPARATORS:

    Electrical comparators are also known as electro- mechanical measuring systems as these employ an electro- mechanical device which converts a mechanical displacement into electrical signal.

Block Diagram of Electro-Mechanical Measuring System

Linear Variable Differential Transformer (LVDT)

Phase referenced and absolute value of output voltage of LVDT

TYPE OF ELECTRICAL COMPARATORS

Electroshock Gauges Electricator Gauges Electro limit gauges

ADVANTAGES OF ELECTRICAL COMPARATORS

1. The electrical comparators have got small number of moving parts

2. It is possible to have a very high magnification and the same instrument may have two or more magnifications. Thus the same instrument can be used for various ranges.

3. The mechanism carrying the pointer is very light and not sensitive to vibrations.

4. As the instrument is usually operated on A.C. supply, the cyclic vibration substantially reduces errors due to sliding friction.

5. The measuring unit can be made very small and it is not necessary that the indicating instrument be close to the measuring unit, it can be remote also.

DISADVANTAGES OF ELECTRICAL COMPARATORS:

It requires an external agency to operate

i.e., the A.C. electrical supply. Thus the

variations in the voltage or frequency of

electric supply may affect the accuracy.

2. Heating of coils in the measuring unit

may cause zero drift and alter the

calibration.

3.This is usually more expensive than

mechanical instrument.

PNEUMATIC COMPARATOR Systems of Pneumatic Gauges:-

Based on physical phenomena on which the operation of pneumatic gauges is based, these may be classified as:

1. Flow or Velocity type.2. Back pressure type.

CHARACTERISTICS: Air gauging has rapidly increased during some past time

due to the following important characteristics:  Very high amplifications are possible. It can be used to measure diameters, length, square ness, parallelism, concentricity, taper, centre distance between holes and other geometric conditions. As no physical contact is made either with the setting gauge or the part being measured, there is no loss of accuracy because of gauge wear. For this reason, air spindle and air snap gauges last very long. Also very soft parts which are easily scratched, can be gauged. Internal dimensions can be readily measured not only with respect to tolerance boundaries but also geometric form. In other words, while measuring a bore it can reveal complete story of size, taper, straightness, camber and bell mouth etc. It is independent of operator skill.

        High pressure air gauging can be done with cleansing of the parts which helps to eliminate errors due to dirt and foreign matter.        gauging pressures can be kept sufficiently low to prevent part deflection.        Dimensional variations throughout the length of shaft or cylinder bore can be explored for out of roundness, taper ness, concentricity, regularity and similar conditions.        Not only it measures the actual size, but it can also be used to salvage oversized pieces for rework or to sort out for selective assembly, i.e., it is suitable both for variable inspection (measurement of size) and attribute inspection (GO and NO GO) gauging and limits.

        It is accurate, flexible, reliable, universal and speedy device for inspecting parts in mass production.

ADVANTAGES OF PNEUMATIC COMPARATORS:-1. The gauging member does not come into contact

with the part to be measured and hence practically no wear takes place on the gauging member.

2. It has usually very small number of moving parts and in some cases none. Thus the accuracy is more due to less friction and less inertia.

3. Measuring pressure of air helps in cleaning the dust, if any, from the part to be measured.

4. It is possible to have very high magnification.

5. It is very suitable device for measuring diameter of holes where the diameter is small compared with the length.

6. It is probably the best method for determining the taperness of the circular bores.

DISADVANTAGES OF PNEUMATIC COMPARATORS:-

1. It requires elaborate auxiliary equipment such as accurate pressure regulator.

2. The scale is generally not uniform.

3. When indicating device is the glass tube, then high magnification is necessary in order to avoid the meniscus errors.

4. The apparatus is not easily portable and is rather elaborate for many industrial applications.

5. Different gauging heads are required for different dimensions.